FIELD OF THE INVENTION
[0001] The present invention relates to an EL element used for illuminating display units,
operation panels or the like in various kinds of electronic apparatus.
BACKGROUND OF THE INVENTION
[0002] EL elements are increasingly used in the sophisticated multi-functional electronic
appliances for illuminating the display units and the operation panels. A conventional
printing type EL element is described with reference to FIG. 2 and FIG. 3.
[0003] FIG. 2 is a cross sectional view of a conventional EL element. The conventional EL
element comprises: a transparent insulating film 1 made of polyethylene terephthalate
or the like material; a light transmitting electrode layer 2 formed by a sputtering
process or an electron beam deposition process covering the whole area of upper surface
of the insulating film, or a light transmitting electrode layer 2 formed by printing
a transparent synthetic resin containing indium tin oxide or the like material dispersed
therein; a light emitting layer 5 formed of a synthetic resin binder 3 containing
phosphor 4 of zinc sulfide or the like materials, which emits light, dispersed therein;
a dielectric layer 6 of synthetic resin binder containing barium titanate or the like
material dispersed therein; a back electrode layer 7 of silver/resin or a carbon/resin
composite formed on the dielectric layer 6; and an insulating layer 8 formed of an
epoxy resin, polyester resin or the like material. The light emitting layer 5, the
dielectric layer 6, the back electrode layer 7 and the insulating layer 8 are overlaid
by printing one after the another on the light transmitting electrode layer 2.
[0004] An EL element mounted in an electronic appliance is driven by an AC voltage supplied
to the light transmitting electrode layer 2 and the back electrode layer 7, the AC
voltage is supplied from a circuit of the electronic appliance (not shown). The phosphor
4 contained in the light emitting layer 5 emits light to illuminate display panel,
LCD or the like of the appliance from a backside of the display.
[0005] When the above-configured EL element emits light in a high humidity environment,
a combination of the humidity in the air and the voltage applied sometimes creates
a carbonized synthetic resin binder in the synthetic resin binder 3 of light emitting
layer 5, which is called a black spot and it impairs the illuminating performance.
In order to prevent it, the phosphor 4 of zinc sulfide is generally covered with a
moisture barrier layer 4A of metal oxides such as aluminum oxide, titanium oxide,
silicon dioxide or the like, and aluminum nitride or the like materials.
[0006] In the conventional EL elements, however, if some of phosphors 4 are coagulated with
each other when they are treated to be covered with the moisture barrier layer 4A,
as shown in FIG. 3(a), the boundary portion 9 between the phosphors 4 may be left
uncovered by the moisture barrier layer 4A. Or, when a mixture of the phosphors 4
and the synthetic resin binder 3 dissolved in a solvent are stirred, the moisture
barrier layer 4A may get damaged as a result of collision between the phosphors 4,
and the phosphor 4 may be exposed as illustrated in FIG. 3(b). Under such circumstance,
there is a problem that the metal ion can elude out from the phosphor 4 in the high
humidity environment, which leads to a deteriorated electrical insulation with the
light emitting layer 5. Thus the black spot phenomenon readily appears.
[0007] To address the above-described problem, the inventors of the present application
proposed in the Japanese Patent Application No. 2000-196109 to disperse a positive
ion exchanger in the light emitting layer 5, so that the ion eluded out of the phosphor
in high humidity environment is captured by the positive ion exchanger contained in
light emitting layer. In this way, the light emitting layer maintains good insulating
property in the high humidity environment even if covering of the phosphor with the
moisture barrier layer is incomplete; thus the black spot becomes difficult to appear.
[0008] The above described improved EL element works well in so far as it is used in the
portable telephone and the like normal electronic apparatus where the voltage applied
is within a range of several volts to twenty volts. However, if it is lit at a high
brightness for a long time driven by a high voltage e. g. several tens or one hundred
volts, the EL element tends to exhibit a problem, or a so-called dark spot. The dark
spot is not seen during OFF time, but when the EL element emits light, some area appears
darker than the surrounding area. This area is called a dark spot. The dark spot phenomenon
is significant among those EL elements in which the light transmitting electrode layer
is formed by a sputtering process and formation of the moisture barrier layer of the
phosphor is insufficient.
[0009] The present invention aims to address the above problem, and provides an EL element
of an improved illuminating property where generation of the dark spot is suppressed,
besides the suppression of the black spot.
SUMMARY OF THE INVENTION
[0010] An EL element of the present invention comprises: a light transmitting substrate;
a light transmitting electrode layer formed on the substrate; a light emitting layer
containing positive ion exchanger; a dielectric layer and a back electrode layer.
A dielectric insulation layer is further provided, between the light transmitting
electrode layer and the light emitting layer, with a dielectric insulation layer being
formed of a synthetic resin that is insoluble with the synthetic resin binder forming
the light emitting layer.
[0011] The present invention provides an EL element of improved illuminating property, with
which the generation of the dark spot is well suppressed, besides the suppression
of the black spot.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012]
FIG. 1 shows a cross sectional view of an EL element in accordance with an exemplary
embodiment of the present invention.
FIG. 2 shows a cross sectional view of a conventional EL element.
FIGs.3(a) and 3(b) show a partial cross sectional view of conventional phosphors.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] Exemplary embodiments of the present invention are described with reference to FIG.
1. Those constituent portions having the same structure as those of the conventional
EL element are represented with the same numerals, and detailed description of which
are eliminated.
First Embodiment
[0014] FIG. 1 is a cross sectional view of an EL element in accordance with an exemplary
embodiment of the present invention. The basic elements of the EL element include
a light transmitting insulating film 1 made of polyethylene terephthalate, polyimide
or the like, a light transmitting indium tin oxide electrode layer 2 formed by a sputtering
process or an electron beam deposition process covering the whole area of the upper
surface of the light transmitting insulating film 1, and a light emitting layer 11
made of a fluoro-carbon rubber or the like synthetic resin binder 3 containing a phosphor
4 of zinc sulfide or the like materials, which emits light, dispersed therein.
[0015] The phosphor 4 is covered with a moisture barrier layer 4A, which is formed of metal
oxides such as aluminum oxide, titanium oxide, silicon dioxide or the like, or formed
of aluminum nitride or the like materials. The light emitting layer 11 contains, in
addition to the phosphor 4, a positive ion exchanger 12 such as an antimonic acid,
phosphoric acid salts, silicic acid salts, zeolite or the like materials, dispersed
therein.
[0016] The light transmitting dielectric insulation layer 13 is formed using a resin material
such as a cyano resin derivatives or a cyano resin derivatives containing high dielectric
constant inorganic particles having a dielectric constant higher than 100. The above
resin material shall be insoluble with the synthetic resin binder forming the light
emitting layer.
[0017] The dielectric insulation layer 13 in the present exemplary embodiment is provided
by printing method between the light transmitting electrode layer 2 and the light
emitting layer 11, for a thickness of 0. 1- 20 µm.
[0018] On the light emitting layer 11, a dielectric layer 6 formed of a high dielectric
constant synthetic resin binder containing barium titanate or the like high dielectric
constant inorganic filler dispersed therein, a back electrode layer 7 of silver/resin
or a carbon/resin composite and an insulating layer 8 of epoxy resin, polyester resin
or the like are further provided by a printing method one after another overlaid in
this order. An EL element is thus structured.
[0019] An EL element of the above configuration mounted in an electronic appliance is driven
by an AC voltage supplied to the light transmitting electrode layer 2 and the back
electrode layer 7. AC voltage is supplied from a certain specific circuit of the electronic
appliance (not shown). The phosphor 4 in the light emitting layer 5 emits light to
illuminate a display panel, such as LCD or the like of the appliance from the backside
of them.
[0020] Now in the following, a method of manufacturing the EL elements is described. Characteristics
of the EL element are also described.
[0021] On a 125 µm thick insulating film 1 of polyethylene terephthalate (PET), a 30 nm
thick indium tin oxide layer is formed by a sputtering process to form a light transmitting
electrode layer 2. And, other layers are stacked thereon one after another by a printing
method as follows:
(1) On the light transmitting electrode layer 2, a 1. 6 µm thick dielectric insulation
layer 13 is formed by printing a cyanoethyl pluran resin ("CR-M" by Shin-etsu Chemical
Industries Co. Ltd) paste dissolved in N-methyl pyrrolidone for a 30% solid content,
using a 350 mesh stainless steel screen mask, and then drying it at 100° C for 30
min.
Besides the above, other samples were manufactured for 10 different layer thickness
with respect to the dielectric insulation layer 13, by varying the solid content of
the cyanoethyl pluran resin, mesh number of the screen, and repeating times of the
printing process (samples No. 1 - No. 10 in Table 1).
(2) On the dielectric insulation layer 13, a synthetic resin binder paste dissolved
in 2-ethoxy-ethoxy-ethanol is printed, and dried at 100° C for 30 min. to form a light
emitting layer 11. The paste includes 100 parts of fluoro-carbon rubber ("Bylon" by
du'Pont), 30 parts of antimony pentoxide hydrate powder (as the positive ion exchanger
12), which are dispersed by a roll mill. And a 50g of the dispersion and a 200g of
the phosphor 4 covered with an aluminum nitride moisture barrier layer 4A ("ANE430"
by Osrum Sylvania) are mixed and agitated together. The paste is screen printed using
a patterned 200 mesh stainless steel screen mask.
Besides the above-described paste, other samples were manufactured also with respect
to the light emitting layer 11 varying the weight % of positive ion exchanger 12 (sample
No. 5 and No. 11 through No. 19 in Table 2).
(3) On the light emitting layer 11, a dielectric layer 6 is formed by printing a dielectric
paste using a patterned 100 mesh stainless steel screen mask, and drying it in the
same conditions as the light emitting layer 11. The dielectric paste is manufactured
with a 22 parts of fluoro-carbon rubber ("Byton A" by E.I. du'Pont) dissolved in 2-ethoxy-ethoxy-ethanol,
and a 78 parts of barium titanate powder ("BT-05" by Sakai Chemical), as a high dielectric
constant inorganic filler, dispersed therein.
(4) On the dielectric layer 6, a back electrode layer 7 is formed by printing a carbon
paste ("DW-250H" by Toyobo) using a patterned 200 mesh stainless steel screen mask,
and drying it at 155°C for 30 min.
(5) Finally, an insulating resist ("XB-804" by Fujikura Kasei Co. Ltd) is printed
using a patterned 200 mesh stainless steel screen mask, and it is dried at 155°C for
30 min. to form an insulating layer 8.
[0022] The sample EL elements No. 1 - No. 10 thus manufactured were evaluated with respect
to the items shown in Table 1.
[0023] The initial brightness (Cd / m
2) was measured by lighting the samples by applying a voltage of 100V, 400Hz, after
they had been put on shelf for one day after they had prepared.
[0024] The brightness maintenance rate was calculated by measuring the brightness after
1000 hours of continuous lighting by 100V, 400Hz in a 25°C, 65% RH humidity chamber,
the brightness was measured 30 minutes after the samples were taken out of the chamber,
and comparing the values with the initial values.
[0025] The dark spot was evaluated by a visual inspection based on the criteria below :
G (no dark spot), F (only a slight dark spot), P (dark spot appears as an unevenness),
B (dark spots covers whole surface making an unevenness).
Table 1
No. |
Dielectric insulation layer (µm) |
Ion exchanger added (wt%) |
Initial brightness (Cd/m2) |
Brightness maintenance rate (%) |
Dark spot evaluation |
1 |
0 |
30 |
96.5 |
38 |
B |
2 |
0.06 |
30 |
96.6 |
39 |
B |
3 |
0.18 |
30 |
97.1 |
42 |
P |
4 |
0.8 |
30 |
96.2 |
51 |
F |
5 |
1.6 |
30 |
95.5 |
54 |
G |
6 |
2.8 |
30 |
94.8 |
54 |
G |
7 |
5.2 |
30 |
91.5 |
56 |
G |
8 |
12.6 |
30 |
81.2 |
61 |
G |
9 |
16.3 |
30 |
68.1 |
63 |
G |
10 |
28.1 |
30 |
32.1 |
71 |
G |
[0026] As Table 1 shows, when compared with sample No. 1 which has no dielectric insulation
layer 13 and sample No. 2 which has a dielectric insulation layer thinner than 0.1
µm, samples having the thicker dielectric insulation layer 13 exhibit the better evaluation
in dark spot and the higher brightness maintenance rate, or the less brightness decrease.
[0027] However, with the increasing layer thickness in dielectric insulation layer 13, the
initial brightness gradually decreases. In the sample No. 10 where the layer thickness
exceeds 20 µm, the initial brightness lowers to approximately 1/3 of the other samples.
[0028] The EL element sample No. 5 and the samples No. 11 through No. 19 underwent a similar
comparative evaluation; the initial brightness (Cd / m
2) by 100V, 400Hz was compared to the brightness after a 240H continuous lighting by
100V, 400Hz in a 40°C, 95% RH humidity chamber for calculating the brightness maintenance
rate, and the black spot was evaluated by a visual inspection based on criteria as
follows: G (no black spot), F (a small number of black spots not greater than 1 mm
φ), P (medium number of black spots not greater than 1 mm φ), B (black spot greater
than 1 mm φ), or a substantial number of black spots not greater than 1 mm φ).
[0029] The result are shown in Table 2.
Table 2
No. |
Dielectric insulation layer (µm) |
Ion exchanger added (wt%) |
Initial brightness (Cd/m2) |
Brightness maintenance rate(%) |
Black spot evaluation |
11 |
1.6 |
0 |
84.1 |
29 |
B |
12 |
1.6 |
0.01 |
83.9 |
32 |
B |
13 |
1.6 |
0.1 |
84.5 |
36 |
B |
14 |
1.6 |
1 |
84.8 |
49 |
P |
15 |
1.6 |
10 |
89.2 |
68 |
F |
5 |
1.6 |
30 |
95.5 |
72 |
G |
16 |
1.6 |
100 |
96.9 |
72 |
G |
17 |
1.6 |
200 |
98.3 |
72 |
G |
18 |
1.6 |
300 |
98.6 |
71 |
G |
19 |
1.6 |
400 |
93.0 |
73 |
G |
[0030] As Table 2 shows, with the dielectric insulation layer 13 with a certain fixed layer
thickness, the brightness maintenance rate goes high along with the increasing quantity
of positive ion exchanger 12 added in the light emitting layer 11; also the black
spot problems improve.
[0031] As described above, if the light emitting layer 11 includes positive ion exchanger
12 and a dielectric insulation layer 13 is provided between the light transmitting
electrode layer 2 and the light emitting layer 11 in accordance with the present embodiment,
the EL elements exhibit an improved illuminating performance, in which an occurrence
of the dark spot is suppressed, in addition to a suppression of the black spot.
[0032] Furthermore, if a dielectric insulation layer 13 is formed with a cyano resin derivatives
or a cyano resin derivatives including a high dielectric constant inorganic particle
having a dielectric constant of higher than 100, the dielectric insulation layer 13
becomes to have high dielectric constant, and the applied voltage is concentrated
to the low dielectric constant light emitting layer 11. As a result, a high brightness
EL element can be obtained.
[0033] Furthermore, when the layer thickness of the dielectric insulation layer 13 is controlled
to be within a range of 0. 1- 20 µm, occurrence of the dark spot can be prevented,
and the brightness decrease can also be suppressed.
[0034] Although in the above descriptions cyanoethyl pluran resin was used as an example
of synthetic resin for the dielectric insulation layer 13, cyanoethyl cellulose, or
cyano saccharose and the like polysaccharide synthetic resin may of course be used
instead for making an EL element of the present invention.
[0035] These cyano resin containing a high dielectric constant inorganic particle having
a dielectric constant of higher than 100, for example, such as titanium oxide having
a dielectric constant of 300, barium titanate having a dielectric constant of 300,
barium titaniate zirconate having a dielectric constant of 6000 can be used for the
same purpose.
[0036] In the above descriptions, antimony pentoxide hydrate powder (antimonic acid) was
used as an example for the positive ion exchanger 12 included in the light emitting
layer 11. However, other positive ion exchanger such as titanium phosphate or the
like phosphoric acid salts, a silicic acid salts, zeolite, or "IXE-100 - 400" by Toa-Gosei
Co. Ltd. may of course be used instead. Namely, any compound or mixture, regardless
of inorganic or organic, that has the positive ion exchange function can be used for
the same effects.
[0037] In the above descriptions, Osrum Sylvania's "ANE430" provided with an aluminum nitride
moisture barrier layer 4A was used as an example for the phosphor 4 of the light emitting
layer 11. However, other phosphor covered with metal oxides such as aluminum oxide,
titanium oxide, silicon dioxide or the like, for example, Osrum Sylvania's CJ type,
or other phosphor without having a moisture barrier layer 4A, for example Osrum Sylvania's
#723 may also be used instead for the same purpose.
[0038] Although a fluoro-carbon rubber was used as an example for the synthetic resin binder
3 of the light emitting layer 11 in the above descriptions, other synthetic resin
binders such as a polyester system, a phenoxy resin, an epoxy resin, an acrylic resin
may also be used instead for the same purpose.
[0039] As described above, the present invention provides an EL element having an improved
illuminating performance, where occurrence of the dark spot is suppressed, in addition
to the suppression of the occurrence of the black spot.